This invention relates to electric control and monitoring circuits, and more specifically, to circuits using self-powered current loops to transmit process variables to remote locations. For many applications, due to safety regulations, the connections to the loop must be earth grounded, creating a ground loop and/or defeating the “hazardous to safe area” requiring isolation.
Products have been provided which provide “galvanic” (transformer) isolation which, due to its inherent design, provides a limited isolation to typically 500 VDC/RMS. Due to the transformer oscillator driven design, such isolation products inject a “chopper” noise on the source, creating problems for other electronic components in the loop, as well as being subject to extreme temperatures and humidity. This limits the use of such transformer oscillator driven isolators in many modern process monitor/control environments.
The use of current loops enables the most popular, safe and easy method of transmitting a process variable to a distance, limited only by the electromotive force (EMF) that drives the loop. The simple two-wire connection of the current loop allows for fast and simple interconnection to as many devices in the loop (in series) as desired, limited only by the EMF of the loop.
Traditional current loop isolators are externally powered through AC mains, current loops or direct current voltage. Such traditional isolators are expensive, complex and bulky. In addition, the conventional isolators use galvanic isolation (transformers) that, due to their nature, have limited voltage breakdown and generate noise due to the oscillator technology used, which introduces errors to equipment in the input loop.
It is desirable to provide a current loop isolator which overcomes the disadvantages of the isolation devices mentioned above, and which eliminates the inherent chopper/switching electrical noise generated by existing galvanically (transformer) isolated technologies.